Variable-flow-rate valve mechanism and turbocharger

ABSTRACT

A clearance δ between an inner peripheral surface of an attachment hole of an attachment tongue and an outer peripheral surface of a valve shaft is set to be smaller than an allowable displacement amount λ in an axial direction of a valve with respect to the attachment tongue. When a condition is satisfied in which the outer peripheral surface of the valve shaft comes into contact with a front-side periphery and a back-side periphery of the attachment hole of the attachment tongue, and in which a top surface of a valve body comes into contact with a back surface of the attachment tongue, a waste gate valve is constituted so that a metal washer becomes non-contact with a front surface of the attachment tongue.

CROSS REFERENCE TO RELATED APPLICATIONS

This application is a continuation of U.S. patent application Ser. No.14/743,106 filed Jun. 18, 2015, which is a continuation of InternationalPatent Application No. PCT/JP2014/056842 filed on Mar. 14, 2014, whichclaims priority to Japanese Patent Application No. 2013-084513 filedApr. 15, 2013, the entire contents of each of which are incorporated byreference herein.

BACKGROUND 1. Technical Field

The present disclosure relates to a variable-flow-rate valve mechanismthat opens and closes an opening of a gas-flow-rate variable passage foradjusting a flow rate of exhaust gas supplied to a turbine wheel side ina turbocharger such as a vehicular turbocharger, and a turbochargerincluding the variable-flow-rate valve mechanism.

2. Description of the Related Art

As measures for preventing an excessive rise in a turbocharging pressureby a vehicular turbocharger (hereinafter, simply referred to as aturbocharger), usually, the turbocharger is provided with a bypasspassage and a waste gate valve. The bypass passage is formed inside aturbine housing of the vehicular turbocharger, and bypasses a part ofexhaust gas from a turbine wheel (a turbine impeller). The waste gatevalve is provided at an appropriate position of the turbine housing, andopens and closes an opening of the bypass passage. Here, the bypasspassage is one of gas-flow-rate variable passages that adjust a flowrate of the exhaust gas supplied to the turbine wheel side. The wastegate valve is one of variable-flow-rate valve mechanisms that open andclose an opening of the gas-flow-rate variable passage.

A general configuration of the waste gate valve will be explained. Asupport hole is formed by penetrating through an outer wall of theturbine housing. A stem is rotatably supported by the support hole. Abase end (one end) of the stem projects outside the turbine housing. Inaddition, a link member is provided integrally with the base end of thestem. The link member swings around an axial center of the stem innormal and reverse directions by driving actuator.

As shown in FIG. 1, an attachment member 153 is provided integrally witha tip (the other end) of the stem. An attachment hole 155 is formed bypenetrating through the attachment member 153. In addition, a valve 157is provided fitted in the attachment hole 155 of the attachment member153. The valve 157 is allowed to have backlash to the attachment member153 (an inclination to a center line C′ of the attachment hole 155 ofthe attachment member 153 is included). Furthermore, the valve 157includes: a valve body 161 that can abut against and separate from avalve seat on the opening side of the bypass passage; and a valve shaft163 provided integrally with a center of the valve body 161, and fittedin the attachment hole 155 of the attachment member 153. Here, backlashof the valve 157 to the attachment member 153 is allowed, and thusfollowability (adhesion) of the valve 157 (valve body 161) to the valveseat on the opening side of the bypass passage is secured. Furthermore,a clasp 167 for making the valve 157 non-detachable from the attachmentmember 153 is provided integrally with a tip of the valve shaft 163.

Accordingly, when the turbocharging pressure (a pressure on an exit sideof a compressor wheel (a compressor impeller)) reaches a settingpressure during operation of the turbocharger, the link member swings inthe normal direction (one direction) by driving the actuator. The stemrotates in the normal direction by the swing of the link member in thenormal direction. The valve swings in the normal direction by rotationof the stem in the normal direction, and the valve body 161 separatesfrom the valve seat on the opening side of the bypass passage. As aresult, the opening of the bypass passage is opened by a waste gatevalve 137, a part of exhaust gas is bypassed from the turbine wheel, anda flow rate of the exhaust gas supplied to the turbine wheel sidedecreases.

In addition, when the turbocharging pressure becomes less than thesetting pressure after the opening of the bypass passage is opened, thelink member swings in the reverse direction (the other direction) bydriving the actuator. When the stem rotates in the reverse direction bythe swing of the link member in the reverse direction, the valve 157swings in the reverse direction, and the valve body 161 abuts againstthe valve seat on the opening side of the bypass passage. As a result,the opening of the bypass passage is closed by the waste gate valve 137,and the flow rate of the exhaust gas supplied to the turbine wheel sideincreases.

Note that Japanese Patent Laid-Open Publication No. 2009-236088 andJapanese Patent Laid-Open Publication No. 2008-101589 show conventionaltechnologies in relation to the present disclosure.

SUMMARY

In order to reduce a chattering sound from a waste gate valve (a contactsound along with vibration of the valve) during operation of theturbocharger, it is necessary to increase not only the weight of thevalve but also the weight of a clasp, and suppress the vibration of thevalve (vibration of the valve and the clasp) by a pulsating pressure ofexhaust gas, or the like.

In contrast, when an outer diameter of the clasp becomes larger due tothe increase of weight of the clasp, the following problem arises. FIG.1 shows a state where the opening of the bypass passage is opened, andwhere the valve 157 is inclined to the center line C′ of the attachmenthole 155 of the attachment member 153. As shown in FIG. 1, when thevalve 157 inclines, the valve body 161 comes into contact with a backsurface 153 b of the attachment member 153, and at the same time, theclasp 167 comes into contact with a front surface 153 a of theattachment member 153. As a result, a large reaction force acts on theclasp 167 from the attachment member 153. Accordingly, a coupling force(a fastening force) of the tip of the valve shaft 163 and the clasp 167is reduced along with increase in an accumulated operation time of theturbocharger, and it becomes difficult to enhance durability of thewaste gate valve, namely, durability of the turbocharger. Note thatround marks of FIG. 1 indicate the above-described contact portions.

That is, there arises a problem in which it is difficult to enhancedurability of the turbocharger while reducing the chattering sound fromthe waste gate valve during operation of the turbocharger.

Note that the above-described problem arises not only in the waste gatevalve but also similarly in other variable-flow-rate valve mechanismsused for turbochargers such as a vehicular turbocharger.

Consequently, an object of the present disclosure is to provide avariable-flow-rate valve mechanism and a turbocharger which can solvethe above-described problem.

A first aspect of the present disclosure is a variable-flow-rate valvemechanism used for a turbocharger in which a gas-flow-rate variablepassage for adjusting a flow rate of exhaust gas supplied to a turbinewheel side has been formed inside a turbine housing or inside aconnection body connected in a state of communicating with the turbinehousing, and that is configured to open and close an opening of thegas-flow-rate variable passage, the variable-flow-rate valve mechanismcomprising: a stem rotatably supported by a support hole formed bypenetrating through an outer wall of the turbine housing or theconnection body, and having a base end projecting outside the turbinehousing or the connection body; a link member provided integrally withthe base end of the stem, and configured to swing around an axial centerof the stem in normal and reverse directions by driving an actuator; anattachment member provided integrally with the stem, and provided withan attachment hole formed penetrating through the attachment member; avalve including: a valve body fitted in the attachment hole of theattachment member with being allowed to have backlash to the attachmentmember, and configured to abut against and separate from a valve seat onan opening side of the gas-flow-rate variable passage; and a valve shaftprovided integrally with a center of the valve body, and is fitted inthe attachment hole of the attachment member; and a clasp providedintegrally with a tip of the valve shaft, configured to make the valvenon-detachable from the attachment member, wherein a clearance betweenan inner peripheral surface of the attachment hole of the attachmentmember and an outer peripheral surface of the valve shaft is set to besmaller than a value obtained by subtracting a depth dimension of theattachment hole of the attachment member from an interval dimensionbetween a top surface of the valve body and a back surface of the clasp,and wherein when a condition is satisfied in which the outer peripheralsurface of the valve shaft comes into contact with a front-sideperiphery and a back-side periphery of the attachment hole of theattachment member, and in which the top surface of the valve body comesinto contact with a back surface of the attachment member, thevariable-flow-rate valve mechanism is constituted so that the claspbecomes non-contact with a front surface of the attachment member.

Here, the “gas-flow-rate variable passage” means inclusion of a bypasspassage for bypassing a part of exhaust gas from the turbine wheel. The“variable-flow-rate valve mechanism” means inclusion of a waste gatevalve that opens and closes an opening of the bypass passage. Inaddition, the “connection body connected in a state of communicatingwith the turbine housing” means inclusion of a piping, a manifold, acasing, and the like which are connected in a state of communicatingwith a gas inlet or a gas outlet of the turbine housing. Furthermore,the “backlash to the attachment member” means inclusion of inclinationand slight movement to a center line of the attachment hole of theattachment member. Being “provided integrally” means inclusion of beingformed integrally. Additionally, the “front-side periphery”, the“back-side periphery”, the “front surface”, and the “back surface” meanportions serving as the front-side periphery, the back-side periphery,the front surface, and the back surface, respectively, when thevariable-flow-rate valve mechanism is planarly viewed from the claspside.

A second aspect of the present disclosure is a turbocharger thatturbocharges air supplied to an engine by utilizing energy of exhaustgas from the engine, the turbocharger including the variable-flow-ratevalve mechanism according to the first aspect.

According to the present disclosure, since a large reaction force doesnot act on the clasp from the attachment member in a state where theopening of the gas-flow-rate variable passage is opened even though aweight of the clasp is increased, reduction of a coupling force (afastening force) of the tip of the valve shaft and the metal washeralong with increase in an accumulated operation time of the turbochargeris sufficiently suppressed while reducing a chattering sound from thevariable-flow-rate valve mechanism during operation of the turbocharger,and durability of the variable-flow-rate valve mechanism, namely,durability of the turbocharger can be enhanced.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a view explaining a problem to be solved by the disclosure.

FIG. 2A is a plan view of a waste gate valve according to an embodimentof the present disclosure.

FIG. 2B is a cross-sectional view along a line IIB-IIB in FIG. 2A, andFIG. 2C is a cross-sectional view along a line IIC-IIC in FIG. 2B.

FIG. 3A is a partial cross-sectional view of the waste gate valveaccording to the embodiment of the present disclosure.

FIG. 3B is a partial cross-sectional view of the waste gate valveaccording to the embodiment of the present disclosure showing a statewhere a valve is inclined to a center line of an attachment hole of anattachment tongue.

FIG. 3C is a partial cross-sectional view of a waste gate valveaccording to a comparative example showing a state where a valve isinclined to a center line of an attachment hole of an attachment tongue.

FIG. 4 is a cross-sectional view along a line IV-IV in FIG. 5.

FIG. 5 is a front view of apart of a vehicular turbocharger according tothe embodiment of the present disclosure.

FIG. 6 is a front cross-sectional view of the vehicular turbochargeraccording to the embodiment of the present disclosure.

DESCRIPTION OF THE EMBODIMENTS

Embodiments of the present disclosure will be explained with referenceto FIGS. 2 to 6. Note that a reference character “L” in FIGS. 4 to 6denotes a left direction, and that a reference character “R” thereindenotes a right direction.

As shown in FIG. 6, a vehicular turbocharger (hereinafter, referred toas a turbocharger) 1 according to the embodiment of the presentdisclosure turbocharges (compresses) air supplied to an engine(illustration is omitted) by utilizing energy of exhaust gas from theengine. Additionally, a specific configuration of the turbocharger 1 isas follows.

The turbocharger 1 includes a bearing housing 3. A pair of radialbearings 5 and a pair of thrust bearings 7 are provided in the bearinghousing 3. A rotor shaft (a turbine shaft) 9 extending in a horizontaldirection is rotatably provided at the plurality of bearings 5 and 7. Inother words, the rotor shaft 9 is rotatably provided in the bearinghousing 3 via the plurality of bearings 5 and 7.

A compressor housing 11 is provided on a right side of the bearinghousing 3. In addition, a compressor wheel (a compressor impeller) 13 isrotatably provided in the compressor housing 11.

The compressor wheel 13 is coupled concentrically integrally with aright end (one end) of the rotor shaft 9, and compresses the air byutilizing a centrifugal force.

An air introduction port (an air introduction passage) 15 forintroducing the air is formed on an entrance side (an upstream side in aflow direction of the air) of the compressor wheel 13 in the compressorhousing 11. The air introduction port 15 is connected to an air cleaner(illustration is omitted) that purifies air. In addition, a diffuserflow passage 17 is formed on an exit side (a downstream side in the flowdirection of the air) of the compressor wheel 13 between the bearinghousing 3 and the compressor housing 11. The diffuser flow passage 17 isformed in an annular shape, and raises a pressure of the compressed air.A compressor scroll flow passage 19 is formed inside the compressorhousing 11. The compressor scroll flow passage 19 is formed in a spiralshape so as to surround the compressor wheel 13. The compressor scrollflow passage 19 communicates with the diffuser flow passage 17. An airoutlet (an air discharge passage) 21 for discharging the compressed airis formed at an appropriate position of an outer wall of the compressorhousing 11. The air outlet 21 communicates with the compressor scrollflow passage 19, and is connected to an air supply manifold(illustration is omitted) of the engine.

A turbine housing 23 is provided on a left side of the bearing housing3. A turbine wheel (a turbine impeller) 25 is rotatably provided in theturbine housing 23. The turbine wheel 25 is concentrically coupledintegrally with a left end (the other end) of the rotor shaft 9, andgenerates a rotation force (rotation torque) by utilizing pressureenergy of the exhaust gas.

As shown in FIGS. 4 to 6, a gas inlet (a gas introduction passage) 27for introducing the exhaust gas is formed at an appropriate position ofan outer wall of the turbine housing 23. The gas inlet 27 is connectedto an exhaust manifold (illustration is omitted) of the engine. Aturbine scroll flow passage 29 is formed on an entrance side (anupstream side in a flow direction of the exhaust gas) of the turbinewheel 25 inside the turbine housing 23. The turbine scroll flow passage29 is formed in a spiral shape, and communicates with the gas inlet 27.A gas outlet (a gas discharge passage) 31 for discharging the exhaustgas is formed on an exit side (a downstream side in the flow directionof the exhaust gas) of the turbine wheel 25 in the turbine housing 23.The gas outlet 31 communicates with the turbine scroll flow passage 29.The other gas outlet (gas discharge passage) 33 for discharging theexhaust gas is formed outside in a radial direction of the gas outlet 31in the turbine housing 23. The gas outlet 31 and the gas outlet 33 areconnected to a catalyst (illustration is omitted) that purifies theexhaust gas via a connection pipe (illustration is omitted). Note thatthe gas outlet 31 and the gas outlet 33 correspond to exits of theturbine housing 23.

A bypass passage 35 as a gas-flow-rate variable passage is formed insidethe turbine housing 23. The bypass passage 35 causes a part of theexhaust gas introduced from the gas inlet 27 to bypass the turbine wheel25, and leads it out to the gas outlet 33 side. In other words, thebypass passage 35 adjusts a flow rate of the exhaust gas supplied to theturbine wheel 25 side. A waste gate valve 37 as a variable-flow-ratevalve mechanism is provided at an appropriate position of the turbinehousing 23. The waste gate valve 37 opens and closes an opening of thebypass passage 35.

A specific configuration of the waste gate valve 37 will be explained.As shown in FIGS. 2A, 2B, 2C, 3A, and 4, a support hole 39 is formed bypenetrating through the outer wall of the turbine housing 23. A stem (arotation shaft) 41 is rotatably supported by the support hole 39 via abush 43. A base end (one end) of the stem 41 projects outside theturbine housing 23. In addition, a link member (a link plate) 45 isprovided integrally with the base end of the stem 41 by fillet weldingand the like.

The link member 45 swings around an axial center of the stem 41 innormal and reverse directions by driving actuator 47. The actuator 47has a well-known configuration incorporating a diaphragm (illustrationis omitted) as shown in, for example, Japanese Patent Laid-OpenPublication No. 10-103069, Japanese Patent Laid-Open Publication No.2008-25442 and the like. When a pressure on the exit side of thecompressor wheel 13 reaches a setting pressure, the actuator 47 swingsthe link member 45 in the normal direction (one direction). When thepressure on the exit side of the compressor wheel 13 becomes less thanthe setting pressure, the actuator 47 swings the link member 45 in thereverse direction (the other direction). Note that an electric actuatorby electronic control or a hydraulic actuator by hydraulic pressuredrive may be used as the actuator 47 instead of using an actuatorincorporating a diaphragm.

An attachment member (an attachment plate) 49 is provided integrallywith a tip (the other end) of the stem 41 by fillet welding etc. Theattachment member 49 is located in the turbine housing 23. Theattachment member 49 includes: an attachment sleeve 51 attachedintegrally with the stem 41; and an attachment tongue 53 providedintegrally with the attachment sleeve 51. An attachment hole 55 in awidth across flat shape is formed by penetrating through the attachmenttongue 53.

A valve 57 is provided fitted in the attachment hole 55 of theattachment tongue 53 (the attachment member 49). The valve 57 is allowedto have backlash to the attachment member 49. The backlash includesinclination to a center line C of the attachment hole 55 of theattachment tongue 53. The valve 57 has a flat top surface 61 t.Furthermore, the valve 57 includes: a valve body 61 that can abutagainst and separate from a valve seat 59 on the opening side of thebypass passage 35; and a valve shaft 63 formed integrally with a centerof the valve body 61, and fitted in the attachment hole 55 of theattachment tongue 53. On a side closer to the stem 41 in the valve body61 (a right side in FIGS. 2A, 2B), a cutout 65 is formed along alongitudinal direction of the stem 41. A cross section of a fittingportion of the valve shaft 63 (a portion fitted in the attachment hole55 of the attachment tongue 53) has a shape including a width acrossflat corresponding to the attachment hole 55 of the attachment tongue53. Backlash of the valve 57 to the attachment member 49 is allowed, andthereby followability (adhesion) of the valve body 61 to the valve seat59 on the opening side of the bypass passage 35 is secured.

A metal washer 67 as an annular clasp is provided integrally with a tipof the valve shaft 63 by fillet welding, swaging, or the like. The metalwasher 67 makes the valve 57 non-detachable from the attachment member49. A chamfering 69 is formed at an outer peripheral edge of a backsurface 67 b of the metal washer 67.

Note that the attachment hole 55 of the attachment tongue 53 and a crosssection of the fitting portion of the valve shaft 63 may be formed in acircular shape instead of the width across flat shape. In addition, thevalve shaft 63 may be provided integrally with the center of the valvebody 61 by swaging, or the like, and the metal washer (clasp) 67 maybeformed integrally with the tip of the valve shaft 63, instead of thevalve shaft 63 being formed integrally with the center of the valve body61, and the metal washer (clasp) 67 being provided integrally with thetip of the valve shaft 63 by fillet welding, swaging, or the like.

Subsequently, a main configuration of the waste gate valve 37 of theembodiment of the present disclosure will be explained.

As shown in FIG. 3A, there is fitting clearance (hereinafter, referredto as clearance) δ between an inner peripheral surface of the attachmenthole 55 of the attachment tongue 53 and an outer peripheral surface ofthe valve shaft 63. The clearance δ is set to be smaller than a value λobtained by subtracting a depth dimension H of the attachment hole 55 ofthe attachment tongue 53 (a thickness dimension of the attachment tongue53) from an interval dimension D between the top surface 61 t of thevalve body 61 and the back surface 67 b of the metal washer 67. Thevalue λ is, in other words, an allowable displacement amount in an axialdirection of the valve 57 (a longitudinal direction of the valve shaft63) with respect to the attachment tongue 53. In addition, in theembodiment, a ratio (λ/δ) of the value λ to the clearance δ is set to be3.5 to 6.5. This is because when the ratio (λ/δ) is set to be less than3.5, the backlash of the valve 57 to the attachment member 49 becomesexcessively large, and it becomes difficult to reduce a chattering soundof the waste gate valve 37, and because when the ratio (λ/δ) is set toexceed 6.5, the backlash of the valve 57 to the attachment member 49becomes excessively small, and it becomes difficult to sufficientlysecure followability of the valve body 61 to the valve seat 59 on theopening side of the bypass passage 35.

As shown in FIGS. 3A and 3B, when a condition is satisfied in which theouter peripheral surface of the valve shaft 63 comes into contact with afront-side periphery and a back-side periphery of the attachment hole 55of the attachment tongue 53, and in which the top surface 61 t of thevalve body 61 comes into contact with a back surface 53 b of theattachment tongue 53, the waste gate valve 37 is constituted so that themetal washer 67 becomes non-contact with a front surface 53 a of theattachment tongue 53.

Specifically, for example, as shown in FIG. 3C, a case is assumed wherethe top surface 61 t of the valve body 61 comes into contact with theback surface 53 b of the attachment tongue 53, and where the metalwasher 67 comes into contact with the front surface 53 a of theattachment tongue 53. In this case, an inclination angle α of the valve57 with respect to the center line C of the attachment hole 55 of theattachment tongue 53 is expressed as tan⁻¹{λ/(R₁+R₂)} from a geometricalrelation. In addition, as shown in FIG. 3B, a case is assumed where theouter peripheral surface of the valve shaft 63 comes into contact withthe front-side periphery and the back-side periphery of the attachmenthole 55 of the attachment tongue 53, and where the top surface 61 t ofthe valve body 61 comes into contact with the back surface 53 b of theattachment tongue 53. In this case, an inclination angle β of the valve57 with respect to the center line C of the attachment hole 55 of theattachment tongue 53 is expressed as tan⁻¹{2δ/(H−R₃)} from a geometricalrelation. Additionally, it is when the inclination angle β becomessmaller than the inclination angle α that the metal washer 67 becomesnon-contact with the front surface 53 a of the attachment tongue 53 whenthe condition is satisfied. In other words, it is when a next inequalityholds that the metal washer 67 becomes non-contact with the frontsurface 53 a of the attachment tongue 53.

tan⁻¹{λ/(R ₁ +R ₂)}>tan⁻¹{2δ/(H−R ₃)}  Formula (1)

Here, R₁ denotes a radius of the top surface 61 t of the valve body 61,R₂ denotes a radius of a portion excluding the chamfering 69 in the backsurface 67 b of the metal washer 67, and R₃ denotes a radius ofcurvature of the back-side periphery of the attachment hole 55 of theattachment tongue 53. Note that round marks of FIGS. 3B and 3C indicatethe above-described contact portions.

Furthermore, when the clearance δ and the value λ are sufficientlysmall, Formula (1) can be replaced as the following Formula.

λ/(R ₁+R₂)>2δ/(H−R ₃)  Formula (2)

Note that FIG. 3C is a partial cross-sectional view of a waste gatevalve 71 according to a comparative example, and that in FIG. 3C, thesame portion numbers as corresponding components of the waste gate valve37 are attached to components of the waste gate valve 71.

Subsequently, action and an effect of the embodiment will be explained.

Exhaust gas is introduced from the gas inlet 27, and circulates from theentrance side to the exit side of the turbine wheel 25 via the turbinescroll flow passage 29. By the circulation of the exhaust gas, therotation force (rotation torque) is generated utilizing the pressureenergy of the exhaust gas, and the rotor shaft 9 and the compressorwheel 13 can be rotated integrally with the turbine wheel 25.Accordingly, the air introduced from the air introduction port 15 can becompressed to be discharged from the air outlet 21 via the diffuser flowpassage 17 and the compressor scroll flow passage 19, and thus the airsupplied to the engine can be turbocharged (usual action of theturbocharger 1).

When a turbocharging pressure (a pressure on the exit side of thecompressor wheel 13) reaches a setting pressure during operation of theturbocharger 1, the link member 45 swings in the normal direction (onedirection) by driving the actuator 47, and the stem 41 rotates in thenormal direction. By rotation of the stem 41 in the normal direction,the valve 57 swings in the normal direction, and the valve body 61separates from the valve seat 59 on the opening side of the bypasspassage 35. As a result, the opening of the bypass passage 35 is openedby the waste gate valve 37, a part of exhaust gas introduced from thegas inlet 27 bypasses the turbine wheel 25, and a flow rate of theexhaust gas supplied to the turbine wheel 25 side decreases.

In addition, when the turbocharging pressure becomes less than thesetting pressure after the opening of the bypass passage 35 is opened,the link member 45 swings in the reverse direction (the other direction)by driving the actuator 47, and the stem 41 rotates in the reversedirection. By rotation of the stem 41 in the reverse direction, thevalve 57 swings in the reverse direction, and the valve body 61 abutsagainst the valve seat 59 on the opening side of the bypass passage 35.As a result, the opening of the bypass passage 35 is closed by the wastegate valve 37, and the flow rate of the exhaust gas supplied to theturbine wheel 25 side increases (ordinary action of the waste gate valve37).

The waste gate valve 37 is constituted so that the metal washer 67becomes non-contact with the front surface 53 a of the attachment tongue53, when the clearance δ is set to be smaller than the value λ, and thecondition is satisfied. Therefore, even though an outer diameter of themetal washer 67 is expanded by the increase in weight of the metalwasher 67, contact of the top surface 61 t of the valve body 61 with theback surface 53 b of the attachment tongue 53 and contact of the metalwasher 67 with the front surface 53 a of the attachment tongue 53 do notoccur simultaneously, when the valve 57 is inclined to the center line Cof the attachment hole 55 of the attachment tongue 53 in a state wherethe opening of the bypass passage 35 is opened. In other words, thevalve body 61 and the metal washer 67 do not come into contact with eachother so as to sandwich the attachment tongue 53 in a state where theopening of the bypass passage 35 is opened. As a result, even though theweight of the metal washer 67 is increased, a large reaction force doesnot act from the attachment tongue 53 to the metal washer 67 in a statewhere the opening of the bypass passage 35 is opened. Particularly,since the chamfering 69 is formed at the outer peripheral edge of theback surface 67 b of the metal washer 67, the weight of the metal washer67 can be more increased while promoting the expansion of the outerdiameter of the metal washer 67.

Furthermore, the cutout 65 is formed on the side closer to the stem 41in the valve body 61 along the longitudinal direction of the stem 41.Therefore, even though backlash of the valve 57 to the attachment member49 is small, the valve body 61 can be reliably caused to abut againstthe valve seat 59 on the opening side of the bypass passage 35 by swingof the valve 57 in the reverse direction. In other words, the opening ofthe bypass passage 35 can be reliably fully closed (full closingproperty of the opening can be enhanced) by the waste gate valve 37(unique action of the waste gate valve 37).

Accordingly, according to the embodiment, even though the weight of themetal washer 67 is increased, the large reaction force does not act fromthe attachment tongue 53 to the metal washer 67 in a state where theopening of the bypass passage 35 is opened. Therefore, durability of thewaste gate valve 37, namely., durability of the turbocharger 1 can beenhanced by sufficiently suppressing reduction of a combination force (afastening force) of the tip of the valve shaft 63 and the metal washer67 along with increase in the accumulated operation time of theturbocharger 1 while reducing the chattering sound from the waste gatevalve 37 during operation of the turbocharger 1.

Moreover, reliability of the waste gate valve 37, namely, reliability ofthe turbocharger 1 can be enhanced since the full closing property ofthe opening of the bypass passage 35 can be enhanced by the waste gatevalve 37 even though the backlash of the valve 57 to the attachmentmember 49 is small.

Note that the present disclosure is not limited to explanation of theabove-described embodiment, and can be carried out in various aspects bymaking appropriate change such as providing a waste gate valve(illustration is omitted) that opens and closes an opening of a bypasspassage (illustration is omitted) formed in an exhaust manifold(illustration is omitted), at an appropriate position of the exhaustmanifold connected in a state of communicating with the gas inlet 27 ofthe turbine housing 23, or the like, instead of providing the waste gatevalve 37 that opens and closes the bypass passage 35, at the appropriateposition of the turbine housing 23.

Additionally, the scope of right encompassed in the present disclosureis not limited to the above-described embodiments. Namely, thevariable-flow-rate valve mechanism of the present application is notlimited to the above-described waste gate valve 37, and as shown inJapanese Utility Model Laid-Open No. 61-33923, Japanese Patent Laid-OpenPublication No. 2001-263078, and the like, the variable-flow-rate valvemechanism can also be applied to a switching valve mechanism(illustration is omitted) that switches a supply state and a supply stopstate of exhaust gas with respect to any of a plurality of turbinescroll flow passages (illustration is omitted) formed in a turbinehousing (illustration is omitted). Furthermore, as shown in, forexample, Japanese Patent Laid-Open Publication No. 2010-209688, JapanesePatent Laid-Open Publication No. 2011-106358, and the like, thevariable-flow-rate valve mechanism of the present application can alsobe applied to a switching valve mechanism (illustration is omitted) thatswitches a supply state and a supply stop state of exhaust gas withrespect to any of plural stages of turbine housings (illustration isomitted). Moreover, the variable-flow-rate valve mechanism of thepresent application can also be applied to both embodiments in which aflow rate of exhaust gas supplied to a turbine wheel side is decreasedand increased by opening an opening of a bypass passage (a gas-flow-ratevariable passage). Similarly, the variable-flow-rate valve mechanism ofthe present application can also be applied to both embodiments in whichthe flow rate of the exhaust gas supplied to the turbine wheel side isdecreased and increased by closing the opening of the bypass passage(the gas-flow-rate variable passage).

What is claimed is:
 1. A variable-flow-rate valve mechanism comprising:an attachment member provided with an attachment hole; a valve includinga valve body and a valve shaft provided with the valve body, the valveshaft being fitted in the attachment hole; and a clasp provided at a tipof the valve shaft, wherein a clearance between an inner peripheralsurface of the attachment hole of the attachment member and an outerperipheral surface of the valve shaft is set to be smaller than a valueobtained by subtracting a depth dimension of the attachment hole of theattachment member from an interval dimension between a top surface ofthe valve body and a back surface of the clasp, and wherein when acondition is satisfied in which the outer peripheral surface of thevalve shaft comes into contact with a front-side periphery and aback-side periphery of the attachment hole of the attachment member, andin which the top surface of the valve body comes into contact with aback surface of the attachment member, the variable-flow-rate valvemechanism is constituted so that the clasp becomes non-contact with asurface of the attachment member.
 2. The variable-flow-rate valvemechanism according to claim 1, further comprising: a stem provided withthe attachment member, wherein part of the valve body on a side closerto the stem is cut out along a longitudinal direction of the stem. 3.The variable-flow-rate valve mechanism according to claim 1, wherein aratio of the value obtained by subtracting the depth dimension of theattachment hole of the attachment member from the interval dimensionbetween the top surface of the valve body and the back surface of theclasp to the clearance is set to be 3.5 to 6.5.
 4. Thevariable-flow-rate valve mechanism according to claim 1, wherein achamfering is formed at an outer peripheral edge of the back surface ofthe clasp.
 5. A turbocharger that turbocharges air supplied to an engineby utilizing energy of exhaust gas from the engine, comprising thevariable-flow-rate valve mechanism according to claim 1.